Semiconductor quantum dots (QDs) have recently commanded considerable attention from photonics researchers due to their interesting optical characteristics. Semiconductor QDs are typically nanocrystals (nanocrystal quantum dots, NQDs) that are smaller in size than the diameter of a Bohr exciton in a bulk crystal of the same material. The small dimensions of QDs can, therefore, apply extremely strong quantum confinement to their electronic structures and the optical processes occurring inside. This is reflected by the sharp exciton absorption features and the high photoluminescence (PL) efficiency observed for many compound QDs.
Due to the extremely narrow emission band of monodisperse QDs (FWHM ˜20-30 nm), II-VI compound-based QD-LEDs have produced color-saturated emissions of much higher spectral purities than those of LCDs and OLEDs. The enhanced color purity of RGB QD-LEDs promises unprecedented improvement of the range of color vision in display applications.
Of particular interest to the display industry is developing low-cost, efficient, bright, color-saturated, large-area color displays using QDs. High expectations regarding this type of display result from the superior performance of QD-LEDs with respect to the conventional display devices such as cathode-ray tubes (CRTs), liquid crystal displays (LCDs), or organic light-emitting-diodes (OLEDs).
QD-LED displays are expected to consume much less power than LCD displays because of their high-efficiency and the emissive nature. The QD-LED luminous efficiency has a theoretical limit of 100 lm/W, equivalent to phosphor-based LEDs as opposed to fluorescent devices (25 lm/W). Yet, the luminescent decay of the QD emissive materials is an order of magnitude faster than most phosphorescent molecules, allowing high efficiency QD-LEDs even at very high brightness (such as 1000 cd/m2).
In an LCD display system, a backlight illuminates every pixel on the screen. Dark pixels are created passively by blocking this light, wasting the light energy. Since quantum dots emit light rather than filtering it, a QD-LED display could potentially use only a small fraction of the power needed to operate an LCD screen used in laptops.
There are great challenges to address along the roadmap of QD-LED display technology. In order to fabricate RGB-pixel arrays of bright QD-LEDs over large surface areas for passive/active matrix displays, there is an urgent need for an appropriate technology to efficiently deposit and pattern QD layers, preferably with precise controls over thickness, composition, surface morphology, and resolution.
There remains an urgent need for a suitable technology for QD deposition/patterning for future development of QD-LED based display technology.